Write gitlab shell configs to gitaly's config

fixes: gitaly#2674 (closed)

We are transitioning away from having both config.toml (gitaly application configuration) and config.yml (gitlab shell configuration). Instead, we will consolidate all of gitlab shell's config.yml into gitaly's config.toml. The first step of that is to write the configs to both places.

This MR takes care of writing relevant gitlab shell config values to gitaly's config.toml

Here is the gitaly MR that changes the way gitlab-shell's config is consumed: gitaly!2066 (merged)